I'm convinced that 1) the background noise of two or four jet engines a few yards away and a fuselage hurdling through the wind cover up all but the most prideful evacuations and 2) down by your feet is the air exchange which sucks up most smells immediately.I've flown a lot and can only think of a couple of times I've either heard or smelled some obvious ass drop. This includes the hundreds of times I've farted. This has to be design, as there is no way they could have overlooked that people were going to spend hours in a tube with other people, and most of those people were going to fart.